Mink cell focus-forming murine leukemia virus (MCF-MuLV) is a recombinant between ecotropic and endogenous xenotropic-related MuLV sequences. Some MCF viruses accelerate leukemia development upon intrathymic inoculation into young mice of their strain of origin. To determine the exact nature of the env sequences in the recombinant MCF CI-3 virus acquired in vitro, and to more precisely define the extent of recombinational exchange, as well as the deletion in another MCF virus (CI-4), the envelope gene sequences were determined and compared with those of ecotropic, xenotropic, and dualtropic MuLVs. Comparison of these sequences revealed several important features. (1) The extent of recombinational exchange in CI-3 is from 145 nucleotides 3 feet of the splice acceptor site for the envelope mRNA to nucleotide 1722, between the end of gp70 and the beginning of Prp15E. (2) Comparison of the deduced amino acid sequence of the CI-3 MCF gp70 with that of ecotropic gp70s reveals extensive conservation of the primary sequence in the carboxy half of the molecule, as well as in a centrally located polyproline domain. Two regions of substantive differences are found on either side of a region of partial homology, suggesting that either or both of these two variable regions may encode the receptor specificity and, consequently, the host range of these viruses. A nonidentity of xenotropic and MCF gp70s was observed, suggesting that xenotropic MuLVs are not the nonecotropic parent of the env gene of MCF-MuLVs. (3) The deletion present in the replication-defective CI-4 virus is bordered by an 11-nucleotide direct repeat in CI-3 viral DNA. (4) Comparison of the substituted sequence in CI-3 with that of Moloney (Mo)-MCF suggests a very close relationship, if not identity, between the endogenous dualtropic proviruses from which they were derived. Two majro mechanisms for the formation of MCF-MuLVs may be envisioned; one involves recombination between unintegrated DNA intermediates representing an exogenous viral genome and an endogenous dualtropic proviral sequence. The sequence data is consistent with the env gene recombination event proceeding through the formation of H structures.